Snowmelt acidity

Amphibians such as frogs and salamanders depend heavily on temporary ponds for breeding. These ponds are highly vulnerable to the "acid shock" events associated with storms or snowmelt. In several studies, reproduction of amphibians has been shown to be seriously restricted when acidity of their habitat decreases to a pH value of less than 5 14). 

Rainwater and snowmelt water are primary factors determining the very nature of the terrestrial carbon cycle, with photosynthesis acting as the primary exchange mechanism from the atmosphere. Bicarbonate is the most prevalent ion in natural surface waters (rivers and lakes), which are extremely important in the carbon cycle, accoxmting for 90% of the carbon flux between the land surface and oceans (Holmen, Chapter 11). In addition, bicarbonate is a major component of soil water and a contributor to its natural acid-base balance. The carbonate equilibrium controls the pH of most natural waters, and high concentrations of bicarbonate provide a pH buffer in many systems. Other acid-base reactions (discussed in Chapter 16), particularly in the atmosphere, also influence pH (in both natural and polluted systems) but are generally less important than the carbonate system on a global basis. 

Figure 16. Outflow chemistry from two snowmelt seasons (1986 and 1987) at Emerald Lake, a high-elevation lake in the Sierra Nevada Mountains of California. Maximum N03 concentrations are coincident with ANC minima during the early stages of snowmelt in 1986 and with a rain-on-snow event in 1987. Nitrate episodes are smaller in magnitude than at sites in the eastern United States, hut western lakes may he more susceptible to episodic acidification because they have a lower baseline acid-neutralizing capacity than most eastern lakes. (Reproduced with permission from reference 180. Copyright 1991 American Geophysical Union.)...

In areas with large snowpacks (e.g., much of the Northeast and all of the mountainous West), ions have been shown to drain from the pack in the early stages of snowmelt. This process leads to concentrations that are much higher than the average concentration of the snowpack itself (82). D ifferential elution of acid anions (like N03") during the initial stages of snowmelt has been shown to be responsible for the elevated N03" concentrations observed in parts of Scandinavia (81), Canada (82), the Adirondack Mountains (181), the Midwest (182), and the Sierra Nevada Mountains (180). Ammonium deposited to the snowpack (either wet or dry deposition) can subsequently... 

Some evidence does exist that mechanisms other than atmospheric deposition contribute to N03" episodes, at least on a small scale. Rascher et al. (43), for example, showed that mineralization of organic matter in the soil during the winter months and subsequent nitrification contribute substantially to snowmelt N03 concentrations at one site in the Adirondacks. Schaefer and Driscoll (83) suggested that a similar phenomenon contributes to N03 pulses during snowmelt at 11 Adirondack lakes and that the contribution from mineralization is greater in low-ANC and acidic lakes. Murdoch and Stoddard (127) presented similar results for streams in the Catskill Mountains. Stottlemyer and Toczydlowski (184) also reported that mineralization contributes to snowmelt NO3" at a site on the upper peninsula of Michigan. It is not currently known how widespread this phenomenon is. 

The acidification problem in both the United States and Canada grows in magnitude if episodic acidification (brief periods of low pH levels from snowmelt or heavy downpours) is taken into account. Lakes and streams throughout the United States, including high-elevation western lakes, are sensitive to episodic acidification. In the Mid-Appalachians, the Mid-Atlantic Coastal Plain, and the Adirondack Mountains, many additional lakes and streams become temporarily acidic during storms and snowmelt. Episodic acidification can cause large-scale fish kills. ... 

Campbell PGC, Hansen HJ, Dubreuil B, et al. 1992. Geochemistry of Quebec north shore salmon rivers during snowmelt organic acid pulse and aluminum mobilization. Can J Fish Aquat Sci 49 1938-1952. 

When the snow begins to melt in the spring the bicarbonate-rich water under the pine trees is pushed out by the downward percolating acid snowmelt. As a result, there is alkaline water flow only for a few weeks before the acid snowmelt begins to dominate and neutralizes the alkaline soil solutions. It is during this short period that... 

Hmska J., Laudon H., Johnson C. E., Kohler S., and Bishop K. (2001) Acid/base character of organic acids in boreal streams during snowmelt. Water Resour. Res. 37, 1013-1026. 

Figure 14. East Bear Brook Aluminum. Aluminum concentrations in the field arc relatively low compared to laboratory reactors and small plot studies at more acidic pH values. Peak concentrations are due to snowmelt and runoff events. Data courtesy of Charles Driscoll, Syracuse University and Steve Norton, University of Maine.

Seasonal acidification is the periodic increase in acidity and the corresponding decrease in pH and ANC in streams and lakes. Episodic acidification is caused by the sudden pulse of acids due to spring snowmelt and large rain events in the spring and fall. Increases in nitrate are important to the occurrence of acid episodes and tend to occur when trees are dormant and therefore using less nitrogen. Short-term increases in acid inputs to surface waters can reach levels that are lethal to fish and other aquatic organisms. 

Aldehydes from amino acid chlorination, such as 2-methylpropanal (derived from valine), 2- and 3-methylbutanal (derived from leucine and isoleucine, respectively), and phenylacetaldehyde (derived from phenylalanine), have been found to be potent contributors to taste-and-odor problems in the drinking water supply of Edmonton, Alberta. The incidents occur in the spring following snowmelt and concomitant increases in raw water color and TOC (Hrudey et al., 1988). Interestingly, the aldehydes were also formed during monochloramine disinfection. 

The Chemistry of Weathering—Long-term Control of Acidification SHORT-TERM AND EPISODIC ACIDIFICATION. 4.1 High Discharge from Snowmelt and Rain Pulsed Release of SO4 and NOsfrom Soils Marine Aerosols Organic Acidity... 

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